As is well known, at present, the bodies of many automobiles are monocoque bodies in which a load is supported by the overall body which is integral with a frame in order to achieve both a decrease in weight and high stiffness. The body of an automobile must be able to suppress impairment of the functions of the vehicle at the time of a collision of the vehicle and protect the lives of passengers within a passenger cabin. In order to decrease damage to a passenger cabin by absorbing the energy of impact at the time of a collision of a vehicle and reduce the impact force to the passenger cabin, it is advantageous to preferentially crush spaces other than the passenger cabin, such as the engine compartment or the trunk.
On account of such safety demands, crash energy absorption members which actively absorb impact energy by collapsing when an impact load is applied at the time of a collision are provided in suitable locations, such as at the front, the rear, or the side of a vehicle. Examples of such crash energy absorption members are cross side members, side sills, and rear side members.
In recent years, it has been attempted to increase the safety of vehicles and to reduce repair costs by nearly eliminating damage to vehicles caused by light impacts by mounting a crash energy absorption member referred to as a crush box on the front end of a front side member by a suitable means such as coupling with a mechanical connector or welding. A crush box is a member which absorbs impact energy by preferentially buckling in the axial direction into the shape of a bellows (or accordion) under an impact load which is applied in the axial direction.
Various materials and shapes have thus far been developed for increasing the crash energy absorbing performance of such a crash energy absorption member. The crash energy absorbing performance which is demanded of a crash energy absorption member are, specifically, that it deform into a bellows shape by repeatedly stably buckling in the axial direction when an impact load is applied in the axial direction, that the average load be high at the time of collapse of the crash energy absorption member, and that the maximum reaction force which is generated upon the collapse of the crash energy absorption member be within a range which does not cause damage to other members disposed in the vicinity of the crash energy absorption member.
Up to now, crash energy absorption members which have generally been used have been box-shaped members welded to a backing plate by means of a flange provided on a member having a hat-shaped transverse cross-sectional shape like that disclosed in JP-A 08-128487, for example. In this specification, “flange” means an edge portion which projects outwards from an outline of a transverse cross section.
As a different type, in JP-A 09-277953, a crash energy absorption member is disclosed which decreases the load at the initial stage of a collision and increases the absorbed impact by having a closed cross-sectional structure such that the transverse cross-sectional shape continuously changes from one end towards the other end from a polygon having at least 4 sides to a polygon having a larger number of sides. It is disclosed in JP-A 09-277953 that the initial load becomes too large when the transverse cross-sectional shape of the crash energy absorption member is made a simple polygon.
JP-A 2002-316642 discloses a crash energy absorption member in which a notch is formed in one of the left and right sides or in one of the upper and lower sides at the front end of a prismatic crash energy absorption member having four flat surface portions.
JP-A 2002-139086 discloses a crash energy absorption member in which the maximum load is decreased by providing a crushing bead.
Air bags which in recent years have been mounted on many automobiles as a passenger protection apparatus must start operating with an accurate timing an extremely short period of time after a collision in order to decrease injury to passengers due to the collision. An air bag starts operating in response to a signal which is output based on a change in the impact load which is sensed at the time of a collision by an acceleration sensor mounted on a crash energy absorption member such as a front side member. If the amount of change in the impact load acting on a crash energy absorption member at the time of a collision is not obtained in a stable manner, the output timing of the signal fluctuates, and the air bag can no longer accurately start operating with a desired timing.
JP-A 05-139242 discloses a crash energy absorption member in which a difference is provided in the plate thickness of the front portion and the rear portion of the crash energy absorption member, a step portion is provided between the front portion and the rear portion of the crash energy absorption member so as to produce a difference in cross-sectional area, or a reinforcing member is provided to the rear of the step portion, whereby the impact load is controlled so that the impact load which is absorbed by the crash energy absorption member is divided into two stages. As a result, the energy of a collision is adequately absorbed, and an acceleration sensor can accurately operate at a value close to a set acceleration.